1. Field of the Invention
The present invention relates to a sheet material fixing device which, together with a rotating drum, nips peripheral end portions of a sheet material wound onto a peripheral surface of the rotating drum, and which sucks and fixes the sheet material onto the peripheral surface of the rotating drum by means of negative pressure supplied to sucking grooves formed in the peripheral surface of the rotating drum.
2. Description of the Related Art
One example of the sheet material is a photosensitive planographic printing plate (hereinafter referred to as a xe2x80x9cprinting platexe2x80x9d) which includes a sheet-like substrate, such as a thin plate of aluminum, and a photosensitive layer formed thereon. The size of the printing plate is selected so as to correspond to the size (i.e., length and width) of a material to be printed.
One type of image exposure apparatus which carries out image exposure on a printing plate is structured such that a printing plate is wound onto a rotating drum and is subjected to scanning exposure while being rotated together with the rotating drum, the scanning exposure being carried out by a light beam corresponding to image data being irradiated onto the printing plate. In such an image exposure apparatus, printing plates of various sizes can be used for image printing as long as they are in a predetermined range of sizes.
In such an image exposure apparatus, a fixing device such as a clamp is disposed at the rotating drum and a peripheral end portion of the printing plate is nipped and fixed between the fixing device and the peripheral surface of the rotating drum so as to prevent positional deviation of the printing plate at the time the printing plate is scanned and exposed. When the rotating drum is rotated at a high speed, problems such as xe2x80x9cfloating-upxe2x80x9d of the printing plate may occur due to centrifugal force. In the image exposure apparatus, grooves are formed in the peripheral surface of the rotating drum and the printing plate is sucked and held by means of negative pressure supplied to these grooves so as to prevent the printing plate from rising up in this manner.
If the printing plate is sucked to and held on the outer peripheral surface of the rotating drum in such a manner that the entire surface of the printing plate is made to closely contact the drum, grooves for sucking the printing plate must be provided at least along the end portions of the printing plate. Further, in order to suck printing plates of various sizes, it is necessary to form a large number of grooves so as to correspond to the printing plates of the respective sizes.
However, in case in which the sucking grooves are formed so that printing plates of various sizes (i.e., printing plates having different lengths or widths) can be sucked by the grooves, when a printing plate having a small length or width is to be sucked, the sucking grooves used for sucking large-sized printing plates are not covered by a printing plate and leakage of negative pressure from such uncovered sucking grooves occurs. As a result, the strength of the suction adhesion of the printing plate is lowered due to this leakage of negative pressure.
One method for reliably sucking the printing plate is to provide a large-capacity vacuum reservoir or an aspirator in consideration of the leakage of negative pressure from the sucking grooves which are not covered by a printing plate. However, employing a larger-capacity negative pressure source such as a vacuum reservoir or an aspirator may lead to increases in the size and cost of the apparatus (e.g., an image exposure apparatus) which is equipped with a rotating drum.
In view of the aforementioned, an object of the present invention is to provide a sheet material fixing device in which a sheet material such as a printing plate wound on a the rotating drum can be reliably sucked to and fixed onto the rotating drum, without causing an increase in the size and cost of the apparatus.
A device relating to the present invention is a device for sucking a substantially rectangular sheet which is trained around a rotating drum and fixing the sheet onto the drum, comprising: (a) a chuck disposed along an axial direction of the drum so as to oppose a drum circumferential direction end portion of the sheet, said chuck able to press the end portion of the sheet toward the drum when the sheet is fixed onto the drum; (b) a plurality of sucking grooves formed in an outer peripheral surface of the drum at predetermined intervals in the axial direction of the drum and substantially parallel to each other, said sucking grooves extending along a circumferential direction of the drum; (c) a sucking hole formed at each of the sucking grooves; and (d) a shut-off element which is urged in a direction of shutting-off a corresponding sucking hole, and is able to shut-off the corresponding sucking hole when the shut-off element is not covered by the sheet when the sheet is fixed on the drum.
Another device relating to the present invention is a device for sucking a substantially rectangular sheet which is trained around a rotating drum and fixing the sheet onto the drum, comprising: (a) a chuck disposed along an axial direction of the drum so as to oppose a drum circumferential direction end portion of the sheet, said chuck able to press the end portion of the sheet toward the drum when the sheet is fixed onto the drum; (b) a plurality of sucking grooves formed in an outer peripheral surface of the drum at predetermined intervals in the axial direction of the drum and substantially parallel to each other, said sucking grooves extending along a circumferential direction of the drum; and (c) a partitioning element disposed so as to project toward a corresponding sucking groove, and able to engage with the sheet and partition the sucking groove when the sheet is fixed on the drum.
Yet another device relating to the present invention is a device for sucking a substantially rectangular sheet which is trained around a rotating drum and fixing the sheet onto the drum, comprising: (a) a first chuck which is disposed along an axial direction of the drum so as to face one drum circumferential direction end portion of the sheet, and which can press the one end portion of the sheet toward the drum when the sheet is fixed onto the drum; (b) a second chuck which is disposed along an axial direction of the drum so as to oppose another drum circumferential direction end portion of the sheet, and which can press the other end portion of the sheet toward the drum when the sheet is fixed onto the drum; (c) a plurality of sucking grooves formed in an outer peripheral surface of the drum at predetermined intervals in the axial direction of the rotating drum and substantially parallel with each other, said sucking grooves extending along a circumferential direction of the drum; (d) a sucking hole formed at each of the sucking grooves; (e) a sucking device which is connected to the holes and which may supply negative pressure to the sucking grooves; (f) a shut-off element which is formed at the first chuck and is urged in a direction of shutting-off a corresponding sucking hole, and is able to shut the corresponding sucking hole when the shut-off element is not covered by the sheet when the sheet is fixed on the drum; and (g) a partitioning element which is provided at the second chuck so as to project toward a corresponding sucking groove, and is able to engage with the sheet and partition the sucking groove when the sheet is fixed on the drum.